(a) Field of the Invention
The present invention relates to copper fine powder which has a considerably low electrical resistance in its powdery state, is excellent in packing properties and has sharp particle size distribution as well as a method for preparing the copper fine powder. More particularly, the present invention pertains to copper fine powder used in making a copper paste which is suitably used for forming an electric conductor pattern on a resin substrate mainly used as an electronic circuit, in particular, a, multilayer printed wiring board as well as a method for preparing such copper fine powder.
(b) Description of the Prior Art
Conventionally, copper fine powder has been used as a raw material for the so-called paste to be fired, i.e., a paste which is applied onto a substrate for electronic machinery and tools, such as a glass or ceramic substrate by the screen printing or the direct drawing and then fired to give a thick film.
Recently, there has been desired for the development of such electronic machinery and tools each having a higher packaging density along with the development of high speed, digital electronic machinery and tools for the purposes of miniaturization and weight-reduction thereof and for further functionalization thereof. Nevertheless, circuit boards have conventionally been prepared by the through hole method wherein through holes are formed in a substrate by drilling operations and the substrate is then plated. There are technical limits in these conventional methods and they have not been able to satisfy the foregoing requirements.
As a means for satisfying the foregoing requirements, the development of a multilayer printed wiring board having a via hole (VH) structure has become of major interest lately. In this respect, the via hole structure may cope with not only the reduction in the substrate size, but also automatic design for distributing wires and digital high speed processing of signals.
The multilayer printed wiring board having such a VH structure has been produced by filling the holes formed in the substrate with a solvent free type heat-curable conductive paste which comprises copper fine powder, a resin and a curing agent; sandwiching the substrate between two copper foils; and then subjecting the assembly to heat and pressure. The heat-curable conductive paste used for the production of the printed board must further satisfy such requirements that the copper fine powder as an electrical conductor should have further excellent characteristic properties such as conductivity and packing properties since any evaporation of organic binders and solvents does not cause, unlike the conventional paste to be fired.
With regard to the copper fine powder, various attempts have conventionally been done to improve the copper fine powder so that it has characteristic properties, which make the use thereof in the solvent-containing paste to be fired favorable, for instance, the shape, particle size, particle size distribution and tap density of the copper fine powder, but there have not yet sufficiently been investigated characteristic properties of the copper fine powder which make the use thereof in the solvent-free heat-curable conductive paste favorable. Accordingly, there has not yet been proposed any copper fine powder capable of satisfying the requirements for the characteristic properties of the copper fine powder which permit favorable use thereof in the solvent-free heat-curable conductive paste.
There have conventionally been proposed various kinds of methods for preparing copper fine powder such as mechanical pulverization methods, atomization methods, electrolysis methods, evaporation methods and wet-reduction methods. The wet-reduction methods have been recognized to be preferred methods for preparing copper fine powder for use in the paste to be fired and, in particular, several reduction methods using hydrazine have conventionally been proposed as appropriate means for preparing copper fine powder having a particle size on the order of 0.1 to 100 xcexcm.
Typical examples of such methods are production methods as disclosed in, for instance, Japanese Un-Examined Patent Publication (hereinafter referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) Nos. Hei 2-294414, Hei 4-116109 and Hei 4-235205.
J.P. KOKAI No. Hei 2-294414 discloses a method for preparing copper powder which comprises the steps of adding an alkali hydroxide and a reducing sugar to an aqueous copper salt solution in the presence of at least one compound selected from the group consisting of amino acids and salts thereof, ammonia and ammonium salts, organic amines and dimethylglyoxime to thus precipitate copper suboxide particles; and then reducing the copper suboxide particles with hydrazine.
In addition, J.P. KOKAI No. Hei 4-116109 discloses a method which comprises reducing an aqueous copper salt solution into metal copper particles through copper hydroxide and copper suboxide. In respect of the method, this patent also describes that a reducing sugar and then a hydrazine reducing agent are added to the aqueous copper salt solution after adjusting the pH value of the solution to a level of not less than 12, that the temperature of the reaction solution is controlled to not less than 60xc2x0 C. prior to the addition of the hydrazine reducing agent and that a chelating agent such as a Rochelle salt, an amino acid, ammonia or an ammonium compound is usable to maintain stable dispersion of copper (II) ions in the aqueous solution.
Moreover, J.P. KOKAI No. Hei 4-235205 discloses a reduction method identical to that disclosed in the foregoing J.P. KOKAI No. Hei 4-116109 except that the former further comprises the step of adding a protective colloid to the aqueous solution in portions.
The foregoing Japanese Un-Examined Patent Publications listed above disclose that the copper fine powder prepared by the method disclosed therein is, for instance, characterized in that it has a narrow particle size distribution and a small particle size, but the copper fine powder has still been insufficient for use as a raw material of heat-curable conductive pastes for filling up VH""s since the particle size distribution thereof is still wide and there is observed a bias in the particle size towards the side of smaller size.
Regarding the resin substrate for the multilayer printed wiring board having a VH structure, the existence of the cured resin among the copper fine powder as a conductor in the VH structure becomes a cause of an increase in the electrical resistance , unlike the conventional paste to be fired. In addition, the via holes are filled with a heat-curable conductive paste according to, for instance, a method using a squeegee, but when the particle size of the copper fine powder in the heat-curable conductive paste varies widely, there is observed such a tendency that the via holes are first filled with coarse particles and the heat-curable conductive paste accommodated in the squeegee has thus an increasingly high rate of fine particles in proportion thereto. For this reason, the viscosity of the paste remaining in the squeegee stepwise increases as the number of the substrates to be treated with one batch of the paste accommodated in the squeegee increases and finally the via holes cannot be filled with the paste or insufficiently filled therewith or the paste is adhered to or remains on portions on the substrate other than the via holes.
The copper fine powder used in the heat-curable conductive paste should have the following characteristic properties in order to eliminate or suppress the aforementioned drawbacks:
(1) The copper fine powder should have a sufficiently low electrical resistance as determined in its powdery state;
(2) It should be excellent in the packing characteristics which may ensure the conductivity of the heat-cured conductive paste in the VH;
(3) The content of the copper fine powder in the heat-curable conductive paste can be increased; and
(4) The paste may have an appropriate viscosity, while satisfying the requirement specified in (3).
Accordingly, an object of the present invention is to solve the foregoing problems associated with the conventional copper fine powder as well as the methods for preparing the same and more specifically to provide copper fine powder usable in a heat-curable conductive paste and which can satisfy all of the requirements listed above.
Another object of the present invention is to provide a method for preparing the foregoing copper fine powder which can satisfy all of the foregoing requirements.
The inventors of this invention have conducted various studies to accomplish the foregoing objects, have found out that copper fine powder can satisfy all of the foregoing requirements when it has an electrical resistance as determined in the form of its powdery state, a specific surface area as determined by the BET method, a tap density, a product of the tap density and a particle size which is calculated from the specific surface area, a particle size distribution as determined by the microtrack measurement and a weight loss through hydrogen-reduction each falling within a specific range and that the copper fine powder which satisfies all of the foregoing requirements can be prepared by a specific method and thus have completed the present invention.
According to an aspect of the present invention, there is provided copper fine powder which has an electrical resistance, as determined in the form of its powdery state, of not more than 1xc3x9710xe2x88x923 xcexa9xc2x7cm; a specific surface area, as determined by the BET method, ranging from 0.15 to 0.3 m2/g; a tap density of not less than 4.5 g/cc; a product of the tap density (g/cc) and the particle size (xcexcm), of not less than 13, the particle size (xcexcm) being calculated from the specific surface area according to the following equation:
Particle Size(xcexcm)=6/(8.93xc3x97(specific surface area, as determined by the BET method (m2/g)));
a particle size distribution observed in the microtrack measurement as expressed in terms of D50 and D90 ranging from 4 to 7 xcexcm and 9 to 11 xcexcm, respectively; and a weight loss through hydrogen-reduction of not more than 0.30%.
According to another aspect of the present invention, there is also provided a method for preparing copper fine powder, which comprises the steps of adding an alkali hydroxide to an aqueous copper salt solution containing divalent copper ions maintained at a temperature of not less than 55xc2x0 C. in an amount of not less than the chemical equivalent to form cupric oxide; then gradually adding a reducing sugar to the reaction system while maintaining the temperature of the system to not less than 55xc2x0 C. to reduce the cupric oxide to cuprous oxide; followed by filtration and washing, again converting into a slurry, gradual addition of a hydrazine reducing agent to the slurry in the presence of a pH buffer capable of maintaining the pH to the range of from 5.5 to 8.5 to thus reduce the cuprous oxide to metal copper.